With the size reduction and the functionality multiplication of portable equipment and the sophistication of the Internet communication systems in the recent years, semiconductor elements used for them have an increasing number of terminals and a reduced pitch between terminals. With this tendency, a higher density and a finer wiring are required more than ever before, for a wiring board of a semiconductor package equipped with a semiconductor element.
As a wiring board including fine wirings at a high density, there exists a buildup printed wiring board including a buildup wiring layer, in which wirings are formed at a high density on an outer layer of a base core substrate.
FIG. 6 is a sectional view showing a conventional buildup printed wiring board. As shown in FIG. 6, a conventional buildup printed wiring board includes a base core substrate 73 made of glass epoxy. A through hole 71 is formed in the base core substrate 73 with a drill. A diameter of the through hole 71 is, for example, about 300 μm. Conductor wirings 72 are formed on both sides of the base core substrate 73. Interlayer insulating films 75 are formed so as to cover the conductor wirings 72, respectively. A via hole 74 for connection to the conductor wiring 72 is formed in each of the interlayer insulating films 75. A conductor wiring 76 for connection to the conductor wiring 72 through the via hole 74 is provided on the surface of the interlayer insulating film 75. A wiring board may sometimes be formed as a multilayered wiring board by repeatedly providing an interlayer insulating film including a via hole formed therein and conductor wirings on the conductor wiring 76 as needed.
As a wiring board with further improved operation speed, a thin wiring board at a high density without a core substrate has been proposed. This wiring board is obtained by first forming a wiring layer composed of wirings and an insulating film on a support member such as a metal plate. Thereafter, the support member is removed so that the wiring layer itself serves as a wiring board (for example, see Japanese Patent Laid-Open Publications Nos. 2001-177010, 2002-83893, and 2002-198462). The wiring board is obtained by removing a core substrate from a normal buildup printed wiring board so as to leave a buildup layer alone. Therefore, this wiring board is considered as a ultimate thin wiring board.
Since no through hole is provided in the above-described wiring board, it is not necessary to provide a land serving to connect a wiring to a through hole. Therefore, the control of an impedance is easy while a loop inductance is small. The entire wiring board has high operation speed. Accordingly, the use of the wiring board permits the design of a wiring operating at high speed. Moreover, since a metal plate having excellent evenness, a high elastic modulus, and excellent heat resistance is used as a support member, a fine wiring pattern at a high density with high form stability can be formed by using a high-temperature process.
The wiring board is mainly composed of metal wirings for electric connection and an insulating film for insulating the metal wirings from each other. As an insulating material used for an insulating film in a wiring board including fine wirings at a high density such as a buildup substrates the following materials are conventionally used.
An insulating material made of a thermosetting resin such as an epoxy resin is used for an insulating film of a wiring board fabricated by a semi-additive method or an additive method, that is, a wiring board including a metal wiring formed by electroless plating and/or electrolytic plating. The insulating material is laminated on a substrate in a semi-cured state. After the deposition, the insulating material is heated and cured to be an insulating film. Thereafter, after the formation of a via with a drill or a laser, a treatment such as desmearing is conducted. Then, a metal wiring is formed by electroless plating and/or electroplating.
An insulating material made of a thermosetting resin such as an epoxy resin is also used for an insulating film of a wiring board fabricated by a subtractive method, that is, a wiring board including a metal wiring formed by etching a copper foil. In the subtractive method, an insulating material is applied onto a copper foil to fabricate RCC (Resin Coated Copper foil) in a semi-cured state. The RCC is laminated on a substrate in a semi-cured state. After thermosetting, a via is formed. Thereafter, the copper foil layer is partially etched away, thereby forming a wiring pattern.
Furthermore, as an insulating material, the use of a material containing an inorganic filler in a thermosetting resin such as an epoxy resin and a glass-epoxy composite material obtained by impregnating a medium such as a glass cloth into a resin material such as an epoxy resin is also being examined.
Furthermore, a laminate composed of a plurality of layers is also disclosed as an insulating film of a wiring board. For example, a technique for providing a conductor layer at least on one surface of a polyimide film and providing a bonding layer made of an epoxy resin on the other surface has been disclosed (for example, see Japanese Patent Laid-Open Publication No. 2002-124751) Specifically, in this technique, a laminate composed of a polyimide layer and an epoxy layer is used as an insulating film.
However, the above-described conventional techniques have the following problems. A semiconductor package is normally formed by mounting a semiconductor element made of a semiconductor material such as silicon on a wiring board. The semiconductor element emits heat to have an elevated temperature in operation. When the operation is stopped, the semiconductor element stops emitting heat to have a lowered temperature. When an organic material is used as an insulating material of a wiring board, a thermal expansion coefficient of the organic material is generally several tens of ppm/° C., which is considerably larger than that of silicon (Si) (approximately 4 ppm/° C.). Therefore, with the operation of the semiconductor element, a thermal stress due to a difference in thermal expansion coefficient is generated between the semiconductor element and the wiring board. If the semiconductor element is repeatedly operated and stopped, a thermal stress is also repeatedly applied to the wiring board. As a result, a crack is generated in the insulating layer of the wiring board by the thermal stress. Moreover, if the semiconductor package is used as a vehicle-mounted component or the like, a change in temperature in the environment of use is added to a change in temperature caused with the operation of the semiconductor element because the change in temperature in the environment of use is large. As a result, since the thermal stress is further increased, a crack is more likely to be generated.
In particular, since a thermosetting resin has a small rupture elongation, that is, several % or less, a crack due to a thermal stress is likely to be generated in an insulating film made of a thermosetting resin. Among the thermosetting resins, a generated crack is more likely to expand to cut a metal wiring in an insulating film made of an epoxy resin. As a result, the wiring is broken to put the semiconductor package in an open state. Moreover, the thermal stress causes another problem that a junction interface between a land for connection to BGA and FC and a solder ball is broken. As a result, the metal wiring breakage, the land breakage and the like as described above cause a further problem that the semiconductor element on the wiring board does not normally function.
Moreover, if the insulating film is formed of an epoxy resin alone, it is difficult to treat the epoxy layer by itself because the epoxy resin is fragile with small expansion. Therefore, a film made of an epoxy resin is formed on PET (polyethylene terephthalate) serving as a support member. For use as an insulating film, the support member is peeled off from the epoxy resin film. Therefore, there arises a problem that the formation of a wiring board requires a step of peeling off the support member from the epoxy resin film.
Furthermore, for an insulating material obtained by an inorganic filler contained in a thermosetting resin, a thermal expansion coefficient: can be lowered by adding an inorganic filler in a thermosetting resin such as an epoxy resin to be closer to a thermal expansion coefficient of a semiconductor element so as to reduce a thermal stress because the inorganic material generally has a small thermal expansion coefficient. However, since the rupture elongation and the rupture strength of the entire insulating material are lowered by containing the inorganic filler, sufficient crack resistance cannot be still obtained.
Furthermore, in an insulating material obtained by impregnating the glass cloth material into an epoxy resin, a stress is absorbed by a glass cloth having high strength. However, a part of the insulating material, which does not include any glass fibers, is inevitably generated in terms of a structure of a woven cloth. A crack is generated and a wiring is broken in this part. Moreover, a method of using light (photovia) and a method of using a laser are generally used as a method of forming a via in a wiring board. However, if a wiring board contains glass fibers, the formation of a via with light is not possible. Moreover, for the formation of a via with a laser, the processability with a laser is poor because the melting point of glass is considerably higher than that of an organic material. Therefore, the obtained via is large with a diameter of 100 μm or more. Thus, it is difficult to use the insulating material for a high-density substrate requiring a fine wiring and a fine via. Moreover, since a glass cloth material serving as an inorganic material provide low adhesion with an epoxy resin serving as an organic material, migration is likely to occur through the interface between the glass cloth material and the epoxy resin. Furthermore, since the glass cloth material has a larger specific gravity than the epoxy resin, the glass cloth material is not suitable for a wiring board of equipment that is required to be reduced in weight such as portable equipment.
Furthermore, if the laminate obtained by laminating a bonding layer made of an epoxy resin or the like on a polyimide film disclosed in Japanese Patent Laid-Open Publication No. 2002-124751 is used, the effects of preventing the generation of a crack during the fabrication of a wiring board or at the early stage of a test can be obtained to a certain degree by reducing a linear expansion coefficient of the polyimide film. However, since the tensile rupture strength at a predetermined temperature or the like is not appropriately controlled, the mechanical characteristics of a wiring board gradually degrade to generate a crack if a thermal stress is repeatedly applied to the wiring board. Therefore, the wiring board does not offer long-term reliability.
The enlargement of size, the increase in number of pins and the reduction of a pitch with the higher operation speed and the higher integration of a semiconductor element are expected to rapidly proceed from now on. Thus, the wiring board, on which a semiconductor element is mounted, is required to have a higher density and a finer wiring. Therefore, it is believed that the problem of a crack generated in the insulating film becomes more noticeable from now on. In particular, metal wiring breakage due to a crack generated with a thermal stress becomes a particularly serious problem in a multilayered wiring board without a core substrate disclosed in the above-described Japanese Patent Laid-Open Publications Nos. 2001-177010, 2002-83893, and 2002-198462 because the multilayered wiring board does not include a core substrate for absorbing a generated stress.
The present invention is devised in view of the above problems, and has an object of providing a sheet material suitable as an insulating film of a wiring board such as a buildup wiring board used in a semiconductor package and a wiring board using the sheet material, which have excellent reliability with excellent crack resistance and high adhesion with a substrate or an underlying sheet material.